The present invention relates to an arrangement and to a method for estimating the total noise power at a point in a WDM-system, and in particular for calculating signal output power, i.e. the total optical output power in the absence of noise, from an optical amplifier with the aid of said total noise power, and to use the signal output power to control the channel output power from the amplifier.
As the term implies, Wave Division Multiplexing (WDM) indicates the use of the wavelength domain to increase the information capacity of an optical fibre, in addition to using the time domain. In other words, several different adjacent wavelengths are used to transfer information between network nodes instead of using a single wavelength as in traditional cases. This enables the information carrier capacity to be increased significantly. The signal at each wavelength is sent through the fibre, independent of the other signals, so that the signal will represent a discrete channel of large bandwidth at each wavelength. Capacity depends on the bandwidth of individual channels and on the number of wavelength channels used.
When signals are transmitted over long distances, it is necessary to regenerate or amplify the signals at regular intervals. Optical fibre amplifiers may be used in the case of this latter alternative, these amplifiers possibly comprising an erbium-doped optical fibre that is pumped by a high power semiconductor laser, hereinafter referred to as a pump laser.
The optical amplifier differs from its electric counterpart in that the output power of the optical amplifier is practically constant when the amplifier is in a saturated state, whereas the electrical amplifier has constant amplification. A constant output power presents problems if the number of channels is changed/changesxe2x80x94either intentionally or unintentionally (malfunctioning). It may therefore be desirable to enable the output power to be controlled. The output power of the optical amplifier, and therewith its amplification factor, can be controlled by adjusting the power of the pump laser.
Swedish Patent Specification SE 506403 teaches how this can be achieved with a feedback control circuit in which the channel output power from the amplifier is used as a real value and compared with the desired channel output power, whereafter the error is returned to the pump laser, which duly changes its output power, and therewith also the output power of the amplifier, in accordance with the error.
Swedish Patent Application SE 9703000-1 (unpublished at the time of filing the present patent application) teaches how the channel output power can be obtained in the present context by dividing the total output power of an amplifier by the number of channels that pass through said amplifier. The number of channels is achieved by sending channel number information in a monitoring channel and by adjusting the information in each node, by adding the number of added channels and subtracting the number of dropped channels.
One problem with this solution is that the total output power will contain a certain amount of noise irrespective of how the output power is measured, this noise usually being referred to as ASE-noise (Amplified Spontaneous Emission). This noise is generated primarily in the optical amplifiers. When a plurality of amplifiers are mutually connected one after the other and when the number of channels is small and/or the amplifier input signals are low, the ASE-noise can result in erroneous calculation of the channel output power.
Consequently, the most critical time of a WDM-system is the start-up time, since start-up is normally effected with one channel. In this case, the error may be in the order of magnitude of some dB:s after the signal has passed through a number of amplifiers. The error decreases significantly when the second channel is added and may be tolerable in the case of small networks that include up to ten nodes. However, in the case of point-to-point applications where the distance between the nodes, and thus also between the amplifiers, is greater and the amplifier input signal thus has a lower input power, the output power error will be significant.
Attempts to measure or calculate the ASE-noise have, of course, been made. European Patent Application EP 0 594 178 teaches a single-channel system in which the noise factor, i.e. the added noise in dB:s, is calculated in an amplifier. Downstream of each amplifier in the system there is placed a first bandpass filter which filters out the noise propagated outside the signal wavelength (propagating ASE), i.e. the total noise that has been generated in several amplifiers. Light is tapped off into a second bandpass filters at a point between the amplifier and the first bandpass filter, this second bandpass filter having a wavelength which differs substantially from the signal wavelength. Since this implies that another amplifier that has a corresponding first bandpass filter is located upstream of the first-mentioned amplifier, the light tapped into the second bandpass filter will only contain propagated noise with wavelengths around the signal wavelength, and consequently the propagated noise will be filtered out in this second bandpass filter.
Consequently, the light exiting from the second bandpass filter will be proportional solely to the noise that has been generated in the nearest amplifier. The amplifier input and output powers are then measured and the measurements used to calculate the noise factor, with the aid of a complicated formula.
One problem with the invention described in the aforesaid European Patent Application EP 0 594 178 is that it is not very successful in calculating the channel output power in a WDM-system. This is because if a corresponding first bandpass filter is placed around the wavelengths used in a system that includes several wavelengths, a great deal of the noise propagated between the wavelengths would remain, and since the propagated noise is greater than the noise generated in a single amplifier it would not be particularly useful to calculate the generated noise and subtract solely this noise.
The object of the present invention is to estimate the total noise, i.e. generated noise plus propagated noise, at a point in a WDM-system and primarily at the outlet of an optical amplifier.
Another object of the invention is to use the estimated total noise to calculate the signal output power of an amplifier, i.e. the total output power without noise.
A third object is to use the signal output power to control amplifier channel output powers.
These objects are achieved and problems associated with present technology are solved by realising that the total noise can be estimated as a constant multiplied by the power at a measuring wavelength which is not one of the signal wavelengths but which nevertheless lies within the wavelength band transmitted between the various nodes. The signal output power from an amplifier can then be obtained by reducing the total output power of the amplifier by the constant multiplied by the amplifier output power at said measuring wavelength.
The best optimisation is obtained by choosing the constant experimentally for each amplifier, by activating the amplifier in the system although without transmitting any signal power, and thereafter adjusting the constant until the total output power of the amplifier minus the constant multiplied by the amplifier output power at said measuring wavelength is equal to zero. Experiments have shown that the noise also increases with amplification, and consequently the most optimal procedure is one which also takes amplification into account when adjusting the constant.
However, in the majority of cases, a sufficiently good result is achieved even with a general compensation while using a constant that is chosen conventionally for several amplifications.
One advantage afforded by the invention is that the noise can be estimated in a very simple and inexpensive way.
In one embodiment, the signal output power is used to control the channel output power from an amplifier. This is achieved by dividing the signal output power by the number of channels that pass through the amplifier, so as to obtain a mean channel signal output power, which is consequently noise compensated and which can be used to control the amplifier, as described in Swedish Patent Application SE 9703000-1.
The advantage is that this control of the amplifier is more precise and more reliable than when no compensation for noise is made.